The “subgrade” is the in situ material upon which the pavement structure is placed.  Although there is a tendency to look at pavement performance in terms of pavement structure and mix design alone, the subgrade can often be the overriding factor in pavement performance.

Proper drainage is important to ensure a high quality long lived pavement; moisture accumulation in any pavement structural layer can cause problems.  Moisture in the subgrade and aggregate base layer can weaken these materials by increasing pore pressure and reducing the materials’ resistance to shear.  Additionally, some soils expand when moist, causing differential heaving.   Moisture in the HMA layers can cause stripping because it, instead of the asphalt binder, will adhere to aggregate particles.

A pavement must function within its environment.  The environment can vary greatly across the State at any one time and it can also vary greatly throughout time at any one place.  Environmental variations can have a significant impact on pavement materials and the underlying subgrade, which in turn can drastically affect pavement performance.  The key environmental parameters of concern are typically temperature, frost action and moisture.

Loads are the vehicle forces exerted on the pavement (e.g., by trucks, heavy machinery, airplanes).  Since one of the primary functions of a pavement is load distribution, pavement design must account for the expected lifetime traffic loads.   Loads, the vehicle forces exerted on the pavement (e.g., by trucks, heavy machinery, airplanes), can be characterized by tire loads, axle and tire configurations, load repetition, traffic distribution across the pavement and vehicle speed.